1. Field of the Invention
This invention is in the field of metal-electroplating processes. More particularly, this invention provides a composition for enhancing the life of a bath for electroplating chromium onto a metal substrate.
2. Description of the Prior Art
Chromium-plating baths comprise chromic acid and sulfate ion as the essential ingredients, the sulfate ion generally being provided by sulfuric acid or sodium sulfate, although those are not limiting sources, the requirement being solely that a soluble sulfate be provided. Other catalyst materials to promote plating efficiency are also commonly used.
In the course of the plating operations, some catalysts or other plating additives such as mist suppressants may decompose with the final product being sulfate ions or chemical species which in the chromic acid solution will be oxidized to sulfate. Sulfate is also an impurity in chromic acid and as more and more zero discharge plants are used this sulfate will continuously increase. Sulfate is also frequently dragged into the plating baths from pretreatment operations which occur prior to plating. As the plating process continues and the sulfate concentration in the plating bath increases the chromium metal deposit will become functionally less useful and eventually if the chemical imbalance is not corrected the plating rate will slow to almost zero.
In one approach to solving this problem, Lukens, in U.S. Pat. No. 2,042,611, teaches that a sparingly soluble sulfate such as strontium sulfate is added to the plating bath. The complex equilibrium for this material with chromic acid is such that the solution always has approximately one per cent by weight (wt. %) sulfate relative to the chromic acid concentration. The practical use of this method requires almost daily mixing or stirring of the insoluble materials on the bottom of the plating tank in order to maintain the proper equilibrium concentrations. However, in very deep or exceptionally large tanks this stirring is for practical purposes impossible to achieve. Not infrequently, plating must be halted to allow the particulate materials, which are in the bath as a result of the strontium sulfate and required mixing, to settle back to the tank bottom in order to avoid nodular deposits. Further, plating must sometimes be halted while the entire tank contents are heated once again in order to establish and maintain the complex equilibrium.
Another common method used for controlling sulfate-ion concentration is the addition of barium carbonate to the solution. This method precipitates barium sulfate and forms carbon dioxide and water, and is by far the most common method presently practiced by the chromium-plating industry. The method works well when sulfate-concentration corrections are needed only on an infrequent and small basis.
The addition of each carbonate anion consumes two protons from the solution in its reaction to form water and carbon dioxide. This reaction does not harm the plating bath when it is performed only to a small extent; it will, however, over time raise the pH of the bath, making it inoperative.
Barium carbonate can not be packaged in the same container as chromic acid, since a potentially hazardous reaction would take place if any moisture was present. For this reason, barium carbonate cannot easily be continuously added to the plating bath as a fixed percentage of the chromic acid being added in a conventional replenishment material.